PhD defense of NIMISHA SIVARAMAN

Abstract:
As the number of components in a confined volume is increasing, there is a strong demand for identifying the sources of radiation in PCBs and the prediction of EMC of electronic circuits.   Electromagnetic near field scanning is a general method of identifying the radiating sources in a PCB.  The first part of the thesis consists of the design and characterization of printed circuit magnetic probes with high sensitivity and high spatial resolution. Conventional probes based on microstrip and coplanar configuration is studied.  As the length of the transmission line connected to the probe increases, the probe output contains noise due to common mode voltages which is many induced by the electric field.  In order to suppress the voltage induced due to the electric field, a shielded magnetic probe is designed and fabricated using low cost printed circuit board (PCB) technology. The performance of the passive probe is calibrated and validated   from 1MHz – 1GHz.  The shielded probe is fabricated on an FR4 substrate of thickness 0.8mm and consists of 3 layers with the signal in the middle layer and top and bottom layers dedicated to ground planes. The aperture size of the loop is 800µm x 800µm, with an expected spatial resolution of 400 µm.  The high sensitivity of the probe is achieved by integrating a low noise amplifier at the output of the probe, hence making an active probe. The performance of the shielded probe with different length of transmission lines is made to study. When the probe has to be operated above 100MHz, it is found that the transmission lines connected to the probe should be short (around 1.5cmm). For frequencies below 100MH, the length of the lines can be up to 12cm.   A three axis probe which is able to measure the three components of the magnetic field are also designed and validated by near field scanning above a standard wire over ground structure.
In The second part, the inverse transmission line matrix method (Inv-TLM) method is used reconstruct the source distribution from the near field scan (NFS) data above a single in plane on the PCB.   Even though, the resolution of reconstruction depends on the wavelength and the mesh parameter, the inverse propagation increases the width of the reconstructed wave.  As this method is found to be  ill posed and results in multiple solutions, we have developed a new method based on the two dimensional cross correlation, which represents the near field scan data in terms of the equivalent electric currents of dipole. With the new method, we are able to identify and locate the current sources in the PCB, and is represented with an equivalent source. The method is validated for the current sources with different orientations. The simulated near field data using CST microwave studio is used to validate both the methods. The radiated far field from these equivalent sources is compared with the simulated fields.  The amplitude and phase of the reconstructed equivalent source is a key factor which determines the radiated far field and errors in the predicted field are possible if the amplitude and phase are not correct. With the equivalent dipole electric current, all the sources are represented with positive amplitude. Representation in terms of the equivalent current of magnetic dipole will be more accurate method for PCBs involving many current loops.